When an Evalution-only assignment is created, the associated rubric is processed so that it includes one and only one selection option. But if there's an error during that process, the rubric will end up blank and unusable in the grading app.
There is a straightforward solution:
Go back to the assignment and edit it (click on the assignment, look for the "edit" button in the upper right).
Change the assessment mode to "Evaluation + Feedback" and save. It'll warn you about losing grading data, but that's irrelevant since you weren't able to start grading anyway.
THEN go back and change the assessment mode back to "Evaluation-only". This will re-process the rubric, hopefully without error this time. You should then be good to go.